The mother, Natalya N. Magnitskaya, 59, a computer science teacher, has kept a low profile for nearly 18 months as her son’s employers from the London-based investment fund Hermitage Capital kept the case in an international spotlight. In an interview last week, she said that she felt time was running out to find out what caused his death.

“I want to know what happened to my son,” she said, “even if it is never accepted by the authorities.”

Ms. Magnitskaya, 59, has asked investigators to turn over glass slides and blocks of paraffin that contain blood and tissue samples, a stockpile that has diminished over the course of four successive medical analyses. After investigators refused the request, her lawyer filed an appeal in Moscow on Wednesday, arguing that the decision violated her constitutional rights.

Russian courts rely on government forensic centers rather than independent experts; though a victim’s family may hire a specialist, state investigators have discretion over whether to provide access to case files and tissue specimens.

Mr. Magnitsky, a tax adviser who was accused of helping his employer evade about $17.4 million in taxes, died in November 2009 after spending 11 months in pretrial detention. While in state custody, he was diagnosed with pancreatitis, complained of vomiting and abdominal pain and repeatedly asked for medical treatment. When he was transferred to a clinic hours before his death, an attending physician said he showed signs of psychosis and left him alone in a cell.

Photo

Natalya Magnitskaya, the mother of Sergei L. Magnitsky, at his funeral in a Moscow cemetery in November 2009. Mr. Magnitsky, a tax adviser, died after 11 months in pretrial detention.Credit
Mikhail Voskresensky/Reuters

His death, at age 37, resonated among Moscow’s professional class. President Dmitri A. Medvedev took a personal interest in the case, dismissing a number of prison officials and calling for a thorough investigation.

Nearly 18 months later, however, no suspects or arrests have been announced. An autopsy carried out by a government coroner 12 hours after Mr. Magnitsky died gave the cause of death as sudden heart failure caused by an asymptomatic disease — a finding that largely absolves the authorities of responsibility.

Though Mr. Magnitsky’s family asked for an independent assessment at the time, morgue officials advised against it, said Tatyana N. Rudenko, Mr. Magnitsky’s aunt.

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“They said, first, that it would take a long time to get permission, and second, what is the point if it’s done by the same people in the same system?” she said. “On the same day, they said that we had to take the body out because they had no room for it, and they said the body was decaying.” Mr. Magnitsky’s relatives buried him the following day.

The medical finding has taken on extraordinary importance. In May, Senator Benjamin L. Cardin, Democrat of Maryland, introduced legislation that would impose financial sanctions and visa bans on a list of high-ranking Russian officials implicated in Mr. Magnitsky’s death. The European Parliament has called upon its members to do the same. A retaliatory bill recently submitted to Russia’s State Duma would refuse visas and freeze the assets of any foreigner deemed to have violated the rights of Russian citizens.

“It touches me that people are sympathetic, not having known him at all,” Mrs. Magnitskaya said of the international attention. She described her son as “very nonstandard” — a bookworm who as a toddler would pretend to read thick medical journals, tracing each line with his finger and babbling seriously to himself. In prison, she said, he was so starved for books that he read and reread a women’s magazine from 2005.

She said the repeated extensions of the investigation had left her with dwindling hope that it would end in prosecution.

“I know in our country, when they want something done, it gets done very fast,” she said.

A version of this article appears in print on July 3, 2011, on Page A11 of the New York edition with the headline: Russian Woman Presses Inquiry Into Son’s Death. Order Reprints|Today's Paper|Subscribe